Many crude oils can be described as paraffinic-intermediate type, which precipitate solids with reduction in temperature. Among these oils are the crude oils currently produced, for example, on the North Slope of Alaska, certain crude oils produced in the Gulf of Mexico, and crude oils produced in the Grand County, Utah. The oils from the North Slope of Alaska are currently commingled and blended with gas condensate (resulting from the processing of liberated reservoir solution gas) for transport in the 800-mile, 48-inch diameter, Trans-Alaska Pipeline System (TAPS). The commingled blend is referred to herein as Alaska North Slope (ANS) crude oil.
Heat losses from the warmer crude oil to the environment during transport in the pipeline can result in the reduction of the crude oil temperature, and subsequent formation of a solid phase in the crude oil. The original design of TAPS was based on properties for crude oil from the Prudhoe Bay oil field, blended with gas condensate liquids. This design, for 2 MMBPD maximum pipeline throughput, allowed for short term pipeline operation with high crude oil precipitated solids content during start-up, and for restart of the pipeline after a potential prolonged shutdown during cold mid-winter conditions. The flow of crude oil through TAPS peaked in 1988 at 2 MMBPD and is estimated to be 1.04 MMBPD in 2000. Future crude oil transported in TAPS is expected be a more diverse blend of crude oils from North Slope oil production operations. Other liquids proposed for transport in TAPS include heavy oil, offshore light crude and condensate fields, and Fischer-Tropsch gas-to-liquid (GTL) conversion products. This potential combination of low pipeline flowrates and differing oil types raises questions regarding the amount of wax precipitation which may occur in future TAPS operating modes.
Operating problems relating to the formation of a solid phase in the crude oil transported in TAPS include the following: inability to properly measure standard volumes, increased solids deposition in pipeline and storage tanks, increased liquid viscosity and pumping costs, and the inability to restart the pipeline after a prolonged shutdown in cold conditions due to the yield strength developed by gelled crude oil. It is important to identify the variables, which determine the amount of solids precipitated during crude oil transport as a first step in minimizing detrimental transport effects. These variables are known to include temperature, pressure, composition, and flow rate. Temperature affects liquid-solid equilibria, while pressure effects are minimal due to the low values of Poynting factors for crude oil components. The composition of the crude oil is a primary variable both in terms of light ends (carbon number less than 7) composition of the crude, and the actual content of solid forming components present in the heavy ends (carbon number greater than 25) of the crude oil. Flow rate is believed to affect solid-liquid equilibria by shear-induced modification of wax crystal and micellar-colloidal structures, and the generation of streaming potentials from the flow of charged colloidal particles.
The amount of solid material suspended in crude oil at any given temperature affects the potential for solid deposition. Thus, the ability to predict the appearance temperature and the amount of a solid phase in a crude oil are both essential for the prediction of the amount of solid deposited. This information is also necessary for the selection of solid management practices. The need for a model capable of predicting liquid-solid equilibria for current and future crude oils of a paraffinic-intermediate type character that precipitate solids at reduction of temperature has been identified as an important issue related to the transport of reserves of these oils.
There are analytical methods for the measurement of wax precipitation temperature (WPT), weight percent solid versus temperature, and phase transition thermodynamic properties (i.e., latent heat of fusion). There are also methods that are related and are those used to compare solid deposition by crude oils, and methods which provide qualitative indication of the stability of crude oils with regard to precipitation of solids from crude oil, are also of related interest.
Since the determination of WPT and the amount of solid wax precipitated below WPT are critical for understanding crude oil rheology and solids deposition, there have been many methods proposed for the determination of WPT. These include viscometry, cross polarized microscopy (CPM), differential calorimetry (DSC), densitometry, near-IR spectroscopy (NIR), and acoustic resonance technology (ART). Filtration and centrifugation have been used for determining solid wax content versus temperature for crude oil systems, even through the results are questionable because of occluded oil and there are difficulties for high pressure applications. Pulsed 1H NMR and DSC have also been used, but are problematic and ineffective for low wax crude oils, and crude oils that produce solids of low crystallinity. Infrared spectroscopy has been used to identify solid-solid and solid-liquid phase transitions for alkanes and petroleum waxes, and as an indicator of methylene crystallinity in petroleum derived asphalts.
Infrared spectroscopy methods, such as Fourier transform-infrared (FT-IR) spectroscopy, have been used in crude oil analysis to provide details about specific molecular structures, and characterize precipitated solids. For example, FT-IR has been used to determine the presence of long chain methylene carbons in asphalt materials1. Identification has been made by use of an infrared absorbtion doublet at 720 cm.12 FT-IR has also been used to measure solid-solid, and solid-liquid transition temperatures of certain waxes by measuring the change in intensity with temperature of the 720 cm−1 absorbtion band corresponding to methylene chain rocking.3. This is possible because the 720 cm−1 band is specific to the CH2 rocking vibration7. As disclosed in Snyder et al.8 the infrared band intensities of the 735-715 cm−1 CH2 rocking mode bands of crystalline pure n-alkanes and PE increase with decreasing temperature at a magnitude much larger than predicted from temperature associated changes in density and refractive index.
A near-IR/fiber optics method has also been used to measure the WPT and asphaltene appearance point (AAP) of waxy crude oils. By monitoring the absorbance at 1450 nm wavelength (6896 cm−1 wave number) of a sample of waxy South China Sea crude oil, the WPT of the sample could be determined from a break point in the absorbance curve produced.4 The absorbance of the sample was measured in-situ using a fiber optic probe inserted into a mixed PVT cell apparatus.
Mid-infrared spectroscopy has been used to identify solid-solid and solid-liquid phase transitions for alkanes5,6 and petroleum waxes,3 and as an indicator of methylene crystallinity in petroleum derived asphalts2. It is believed that FT-IR spectroscopy had never been applied to the investigation of wax precipitation.
Measurements of ANS crude oil WPT and weight percent solid versus temperature using conventional methods (i.e., CPM) have been problematic, and in some cases, unsuccessful (i.e., DSC, 1H NMR). For these reasons, there is a need for yet another method for analyzing WPT that can successfully measure WPT for oils like ANS crude oil.